Projectors are widely-used display devices that can be used to produce an image on many surface types. Multiple projectors are commonly used to increase the size of a projection onto a projection surface whilst retaining high resolution and brightness. For example, four projectors can be arranged in a grid configuration to produce a single image that is four times larger than the image produced by a single projector.
A problem of such multi-projector systems is the need for geometric alignment of the projected images on the projection surface. It is important that a viewer perceives a single image that has no visible seams or brightness fluctuations. Precise alignment of the projected images is therefore very important. Many current multi-projection systems require a significant amount of manual effort to perform alignment.
One prior art technique requires the user to establish point correspondences manually. Given two overlapping projections, the user first manually chooses a point in the projection overlap region within the first projector image, and then manually searches and selects a corresponding point within the second projector image.
Some prior art systems perform an automatic alignment procedure at system installation time, for example using projected calibration patterns or structured light patterns, commonly known in the art.
To achieve high accuracy in alignment, an alignment refinement process is needed. Given an initial, approximate coarse alignment, an alignment refinement process can be used to adjust the initial alignment to achieve a more accurate fine alignment.
One prior art technique projects a regular grid like calibration pattern, uses simulated annealing to iteratively estimate alignment parameters and projects a corresponding updated calibration pattern. The alignment parameters are obtained when the algorithm converges. Some drawbacks of this prior art technique are that the calibration pattern is highly visible to human viewers and that the calibration process is slow to update since the technique requires many frames of calibration pattern to be projected, captured and processed.
Multi-projector systems may fall out of alignment over time, for example, due to physical movement of a projector or surface, building vibration, or heat fluctuations causing small movement of a projector's internal components. When such systems become misaligned, a manual or automatic alignment procedure typically needs to be performed.
Other prior art techniques are capable of continuous automatic alignment of multiple projectors. While visible content is being displayed (e.g. a video or image), calibration patterns are also displayed, and are photographed by one or more cameras. The calibration patterns may be in the infrared spectrum. Alternatively, the calibration patterns may be embedded in the projected content images, preferably in such a way that the viewer is unable to perceive the calibration patterns.